1. Field of the Invention
The present invention relates to a cutting point for a drill. More particularly, the present invention relates to a modified s-shaped cutting point for a drill to provide improved thrust and drilling performance.
2. Description of the Related Art
A conventional twist drill comprises a cylindrical drill body provided with a pair of helical flutes defining fluted lands between them and extending from the cutting point of the drill to a shank at the other end, by which shank the drill is securable in a chuck, for example, of a hand tool, or a drilling machine. The cutting point of the drill is of generally conical-shape, with a central chisel edge from which a pair of diametrically opposed cutting edges defined by the leading faces of the fluted lands and the flanks of the drill point that form the end faces of the fluted lands. The outer periphery of each fluted land has at its leading edge a radial projection which is variously termed a land, a cylindrical land, a wear margin, or a support margin. These two support margins extend along the length of their fluted lands and are intended to guide the drill radially as it forms a hole.
It is generally known that the helical grinding of a twist drill in the vicinity of the chisel, or cutting, edge results in a greater clearance angle. This is favorable for easy, centered spot drilling but not favorable for high feed rates, particularly since it weakens the chisel edge. Thus, in order to improve cutting conditions in twist drills having helically ground faces, corrections are often made at the chisel edge, e.g. by making it pointed or by adapting the side rake angle to the material to be drilled, as disclosed in Stock-Taschenbuch by R. Stock AG [Handbook published by the R. Stock AG], Berlin, 3rd Edition of 1979, at page 40.
Briefly, according to this invention, there is provided a cutting point for a drill. The drill includes a body having at least two diametrically opposite flutes. The body further includes a periphery, a web and lands extending from the web and having margins at leading edges of the lands. The flutes are positioned between the land.
The cutting point includes a leading cutting lip and a trailing cutting lip and a central chisel edge extending between the leading and trailing cutting lips and defining two symmetrical drill point halves. Each drill point half includes a primary relief surface, a secondary relief surface and a third relief surface. The primary relief surface is inclined radially outwardly from the central chisel edge to the periphery of the body and downwardly from the leading cutting lip to the secondary relief surface. The secondary relief is inclined radially outwardly from the central chisel edge to the periphery of the body and downwardly from the primary relief surface to a rake surface of the flute. The third relief surface is inclined radially outwardly from the central chisel edge to the rake surface of the flute and downwardly from the central chisel edge to the secondary relief surface. The third relief surface is inclined at an angle greater than the secondary relief surface and the secondary relief surface is inclined at an angle greater than primary relief surface.